The goal of our research is to understand the effects of asymmetric hearing loss in adults and children. Hearing asymmetry occurs, for example, when one ear has profound hearing loss and the other ear has normal hearing. Or, when one ear is profoundly deaf but the better ear has partial hearing. In either case, the imbalanced inputs detrimentally affect communication in everyday life and have other negative consequences. In Aim 1, we identify the sources of variability contributing to outcomes observed in adults and children with unilateral hearing loss. Understanding this variability could improve counseling and treatment options for patients. We will evaluate factors that predict speech recognition in noise and localization by investigating cognitive abilities, associated demographics, and measures of performance. In Aim 2, we assess the hypothesis that deficits in comprehending speech in noise with unilateral hearing loss affect processing in non- auditory cognitive brain regions. We propose a functional imaging study of activity evoked by high and low predictability sentences in the presence and absence of multi-talker babble. We predict that compensating for degraded speech comprehension in noise involves greater activation of cognitive centers in temporo-parietal, inferior and dorsolateral frontal, and cingulate cortex in adults with unilateral hearing loss compared to those with normal hearing. In Aim 3, we examine the benefit of cochlear implantation for adults and children with asymmetric hearing loss and varied ages at onset of deafness and hearing histories in the poorer ear. We predict that even if one ear has partial hearing and the brain has learned to hear with a fairly good acoustic signal, age at onset of deafness in the poorer ear will be associated with outcome. In Aim 4, we identify factors in bilateral cochlear implant recipients that contribute to asymmetry in speech recognition between ears and reduced bilateral performance. In prelingually deaf children, we expect that a longer time between surgeries corresponds to larger asymmetries in speech recognition between ears and reduced bilateral performance. In postlingual adults, we predict that greater divergence in electrode placement between cochleas correlates with larger asymmetries in speech recognition between ears, reduced bilateral performance, and larger interaural differences in pitch percepts. To summarize, the proposed research program will probe the basis of behavioral variability and brain reorganization in unilateral hearing loss. Findings will determine how asymmetry in hearing affects behavioral outcomes and whether new considerations exist for early onset of unilateral deafness. Knowledge gained could influence decisions about treated ear, treatment types, and timing of treatment in children and adults with asymmetric hearing loss. Finally, results will enhance understanding of inter-cochlea electrode placement effects and potentially influence treatment decisions for bilateral implant recipients.